Sym

Comment

Sym

Comment

A

Heat transfer area

${\dot{q}}_{inc-upper}$

Solar radiation on absorber’s upper half

$A_c$

Concentrator’s captation area

${\dot{q}}_{net}$

Net heat absorbed by the absorber

c

Specific heat

${\dot{q}}_{rad-abs}$

Absorber’s radiative heat loss

D

Diameter of absorber tube

${\dot{q}}_{rad-em}$

Emitted radiation

e

Enclosure’s wall thickness

${\dot{q}}_{rad-rec}$

Received radiation

$F_{ij}$

View factor between surfaces “i” and “j”

${\dot{q}}_{rad-we}$

Radiative heat transfer to outer face of enclosure’s wall

g

Gravity

${\dot{q}}_{rad-wi}$

Radiative heat transfer to inner face of enclosure’s wall

h

Convective heat transfer coefficient

${\dot{q}}_s$

Sensible heat of enclosure’s walls

$h_e$

Outer-face convective heat transfer coefficient of enclosure’s wall

${\dot{q}}_{sol}$

Solar irradiance

$h_i$

Inner-face convective heat transfer coefficient of enclosure’s wall

r

Radius of absorber tube

k

Thermal conductivity

R

Concentrator’s reflectivity

L

Characteristic longitude

Ra

Rayleigh number

$L_{opt}$

Collector’s optical losses

$T_a$

Environmental temperature

m

Mass

$T_{abs}$

Absorber’s surface temperature

Nu

Nusselt number

$T_w$

Central temperature of enclosure’s wall

Pr

Prandtl number

$T_{w-ex}$

Outer-face temperature of enclosure’s wall

PTC

Parabolic trough collector

$T_{w-in}$

Inner-face temperature of enclosure’s wall

${\dot{q}}_{conv-abs}$

Absorber’s convective heat loss

$\beta$

Volumetric expansion coefficient

${\dot{q}}_{conv-we}$

Convective heat transfer to outer face of enclosure’s wall

$\epsilon$

Emissivity

${\dot{q}}_{conv-wi}$

Convective heat transfer to inner face of enclosure’s wall

${\epsilon }_{ij}$

Effective emissivity between surfaces “i” and “j”

${\dot{q}}_{dir}$

Direct solar radiation

$\nu$

Kinematic viscosity

${\dot{q}}_{inc}$

Incident solar irradiation

$\sigma$

Stefan-Boltzmann constant

${\dot{q}}_{inc-lower}$

Solar radiation on absorber’s lower half